Process for producing an enamelling steel sheet

ABSTRACT

A process for producing an enamelling steel sheet which comprises tapping molten steel with manganese content not more than 0.40%, lowering the carbon content in the molten steel to not more than 0.01%, adding titanium to the molten steel in an amount enough to obtain a titanium content between 0.01 and 0.3%, preferably between 0.03 and 0.1%, and also adding one or more of rare earth elements such as lanthanum, cerium, neodymium, praseodymium, and samarium (hereinafter called Rem) in total amount between 0.01 and 0.15%, preferably 0.02 and 0.12%, casting the molten steel into ingot or slab, hot rolling and then cold rolling the ingot or slab preferably at a cold rolling rate between 30 and 90% and then annealing the steel sheet.

United States Patent Gondo et al.

PROCESS FOR PRODUCING AN ENAMELLING STEEL SHEET Inventors: Hisashi Gondo; Hiroshi Takechi;

Kazuo Namba, all of Kisarazu; Matsuo Usuda; Kouichi Kawasaki, both of Kimitsu, all of Japan Assignee: Nippon Steel Corporation, Tokyo,

Japan Filed: Mar. 7, 1974 Appl. No.: 448,958

Foreign Application Priority Data Mar. 9, 1973 Japan 48-27169 US. Cl 148/2; 148/12 C; 75/123 Int. Cl. C21D 7/02; C21D 7/13; C21D 9/46 Field of Search 148/12 C, 12 F, 2;

References Cited UNYIED STATES PATENTS FOREIGN PATENTS OR APPLICATIONS 1,192,794 5/1970 United Kingdom Primary ExaminerW. Stallard Attorney, Agent, or FirmToren, McGeady and Stanger [57] ABSTRACT A process for producing an enamelling steel sheet which comprises tapping molten steel with manganese content not more than 0.40%, lowering the carbon content in the molten steel to not more than 0.01%, adding titanium to the molten steel in an amount enough to obtain a titanium content between 0.01 and 0.3%, preferably between 0.03 and 0.1%, and also adding one or more of rare earth elements such as lanthanum, cerium, neodymium, praseodymium, and samarium (hereinafter called Rem) in total amount between 0.01 and 0.15%, preferably 0.02 and 0.12%, casting the molten steel into ingot or slab, hot rolling and then cold rolling the ingot or slab preferably at a cold rolling rate between 30 and 90% and then annealing the steel sheet.

12 Claims, 1 Drawing Figure N w 1 (J O O O O O O Top Middle Bottom I I I US. Patent Dec. 23, 1975 Eozom E ROCESS FOR PRODUCING AN ENAMELLING STEEL SHEET The present invention relates to a process for producing an enamelling steel sheet having remarkable resistance against fish-scale as well as good non-aging property and deep-drawability and yet showing large weight decrease by acid pickling.

It is required for an enamelling steel sheet that it is free from the surface defect due to hydrogen, namely, fish-scale, and it has good adhesion to enamelling glaze. At the same time, the enamelling steel sheet is required to have good workability such as deep-drawing because the enamelling steel sheet is very often subjected to severe forming into bath tubs, tableware, sanitary utensiles and so on and then enamelled. It is also required in case of single application of enamelling glaze that the enamelling steel sheet is free from surface defects due to carbonaceous gas, such as foams and blisters as well Free from the strain due to enamel firing and that it has a large acid-pickling rate from the point of productivity.

In order to meet the above requirements, it has been conventionally known that titanium is added to the steel to fix carbon and nitrogen in the steel so as to render the steel non-aging and to improve resistance against the fish-scale by the titanium content in the steel. In this case, however, the amount of titanium to be added to fix the carbonincreases, the steel hardens and the production cost increases. Thus, in order to eliminate these defects, it has been conventionally practised to lower the carbon and oxygen contents in the steel preliminary by vacuum degassing, or to maintain the'ratio of titaniumto carbon-of Ti/C 10 so as to improve deep-drawability;

The steel sheet produced by the above conventional methods, however, hasa. defect that surface defects due to titanium compounds appear on the steel surface in some cases, thus causing lowering ofthe production yield. Also in case wheretitanium is added to the steel, the weight decreases during the acid pickling treatment conducted as a preliminary treatment for the enamel firing is relatively small so that the time required for the preliminary treatment is necessitated to be longer in case of the single application 'of enamelling glaze. This is undesirable from the point of the productivity.

Therefore, one of the objects of the present invention is to provide a process for producing an enamelling steel sheet having remarkably excellent resistance against the fish-scale and free from the above defects.

The features of the present invention are summarized as under.

1. A process for producing an emamelling steel sheet which comprises tapping molten steel with manganese content not more than 0.40%, lowering the carbon content in the molten steel to not more than 0.01%, adding titanium to the moltenv steel in an amount enough to obtain a titanium content between 0.01 and 0.3%, preferably between 0.03 and 0.1%, and also adding one or more of rare earth elements such as lanthanum, cerium, neodymium, praseodymium, and samarium (hereinafter called Rem) in total amount between 0.01 and 0.15%, preferably 0.02 and 0.12%, casting the molten steel into ingot or slab, hot rolling and then cold rolling the ingot or slab preferably at a cold rolling rate between 30 and 90% and then annealing the steel sheet.

v 0.12% prior to or at the same time of casting the steel into an ingot mold, leaving the steel after casting into the mold until an initial solidified layer is formed, adding titanium to the non-solidified portion so as to obtain titanium content between 0.01 and 0.3%, preferably 0.03 and 0.1%, hot rolling and cold rolling and annealing the steel sheet.

3. A process for producing an enamelling steel sheet which comprises tapping molten steel with a manganese content not more than 0.40%, lowering the carbon content in the steel to not more than 0.01% by vacuum degassing, leaving the steel after casting to ingot mold until an initial solidified layer is formed, adding titanium and Rem to the non-solidified portion in an amount to obtain a titanium content between 0.01 and 0.3%, preferably 0.03 and 0.1%, and a total content of Rem between 0.01 and 0.15%, preferably between 0.02 and 0.12%, casting the steel into ingot, hot rolling and cold rolling the steel preferably with a cold rolling reduction rate between 30 and and then annealing the steel sheet.

The present invention will be described inmore details referring to the attached drawings.

FIG. 1 shows the initial solidified layer thickness of the steel sheet (A) produced according to the present invention.

For the production of the steel according to the present invention, the molten steel is tapped with a manganese content not more than 0.40%. A smaller manganese content is desirable for reducing the strain due to the enamel firing as well as from the point of material quality, and thus its upper limit is defined as 0.40%. Then, the molten steel is subjected to vacuum degassing treatment to lower the carbon content to not more than 0.01%. The reason for limiting the carbon content to not more than 0.01% is to prevent occurrence of the surface defects such as foams and blisters due to the carbonaceous gas and to prevent the strain due to the enamel firing. The oxygen content is also reduced by the degassing, but the remaining oxygen can be further reduced by a preliminary deoxydation with aluminum, for example, after the completion of the degassing treatment, if necessary.

Prior to the pouring the molten steel into the ingot mold or during the pouring, titanium and Rem are added to the molten steel so as to obtain a titanium content between 0.01 and 0.30%, preferably 0.03 and 0.10% and a total Rem content between 0.01 and 0.15%, preferably 0.03 and 0.12%. The reason for limiting the titanium content in the steel between 0.01 and 0.30%, preferably 0.03 and 0.10% is that if the titanium content is more than 0.30%, the steel hardens and press formability is lowered and the production cost increases, while the titanium content is less than 0.01%, no substantial improvement on the resistance against the fish-scale and deep-drawability can be expected. Meanwhile the reason for limiting the Rem content between 0.01 and 0.15%, preferably 0.02 and 0.12% is that a Rem content more than 0.15% will cause increased production cost and, on the other hand, a Rem content less than 0.01% no substantial improvement on the resistance against the fish-scale 3 can be expected.

The steel slab produced according to the present invention is subjected to hot rolling, acid pickling, electrical cleaning, annealing and, if necessary, temper rolling. The hot rolling may be conducted under ordinary conditions, but desirable results can be obtained by conducting the hot rolling with coiling temperature between 450 and 800C, preferably 550 and 730C. If the hot coiling temperature exceeds 800C, it is difficult to adjust the temperature, and on the other hand if the coiling temperature is lower than 450C the improvement effect of the resistance against the fish-scale is reduced.

Regarding the cold rolling, a reduction rate not less than is desirable for obtaining desirable material quality, but with a reduction rate more than 90% the required thickness of the hot rolled steel sheet is too thick for practical use. On the other hand, with a reduction rate less than 30%, desired deep-drawability can not be obtained.

The annealing may be done by any of a box annealing, a continuous annealing and other annealing methods, and the annealing temperature should be between 650 and 1000C, preferably between 650 and 910C in case of the box annealing and an open coil annealing, and between 700 and 980C in case of the continuous annealing. If the annealing temperature is lower than the lower limit, softening of the steel caused by recrystallization is not enough so that desirable workability is not obtained. On the other hand, if the temperature is higher than the upper limit, the amount of transformation into austenite is large so that good deep-drawability can not be obtained, and the higher temperature is not desirable also from the point of the economy of the furnace.

The resistance against the fish-scale of the steel sheet produced according to the present invention defined in paragraph (1) hereinbefore is remarkably good, and it has been found that much larger effect can be obtained as compared with the total combining effect obtained by addition of titanium alone, and effect obtained by addition of Rem alone.

The present inventors have conducted further extensive studies and experiments for solving the problems such as the occurrence of the surface defects due to the titanium compounds and the lowering of the weight decrease during the acid pickling, and have found that these problems can be solved by forming an initial solidified layer containing no titanium on the steel surface according to the present invention as defined in the paragraphs (2) and (3) as hereinbefore stated.

Embodiments of the present invention as defined in the paragraphs (2) and (3) will be described under.

The present invention of the paragraph (2) may be conducted as under.

Molten steel containing not more than 0.40% of manganese preferably not more than 0.25% of manganese is tapped, and subjected to vacuum degassing to reduce the carbon content to less than 0.01%, and then prior to or at the same time of pouring the molten steel into an ingot mold, Rem is added so as to obtain a total Rem content between 0.01 and 0.15%, preferably 0.02 and 0.12%, and the steel is left after the pouring into the mold until an initial solidified layer is formed, and titanium is added to the non-solidified portion so as to obtain a titanium content between 0.01 and 0.3%, preferably 0.03 and 0.10%.

Rem may be added together with titanium to the non-solidified portion of the ingot after the molten steel is cast into the mold and the initial solidified layer containing no titanium and Rem is formed according to the present invention defined in the paragraph 3).

Namely, molten steel with a manganese content not more than 0.40%, preferably not more than 0.25% is tapped and subjected to vacuum degassing treatment to lower the carbon content to not more than 0.01%, and the molten steel is left after it is cast into the mold until an initial solidified layer is formed, and then titanium and Rem are added to the non-solidified portion so as to obtain a titanium content between 0.01 and 0.3%, preferably 0.03 and 0.1%, and a total Rem content between 0.01 and 0.15%, preferably 0.02 and 0.12%.

The initial solidified layer produced by the present invention as defined in the paragraphs (2) and (3) is different from the conventional rim layer formed by the rimming action in their formation mechanisms, and it is the important difference that the initial solidified layer is formed without rimming action, and thus the surface of the ingot produced by the present invention is as good as that of the rimmed steel. Further as the initial solidified layer of the steel produced by the present invention does not contain titanium, surface defects due to the titanium compounds are not caused at all, and the cold rolled steel sheets produced according to the present invention do not contain titanium in their surface layer and thus their weight decrease in acid pickling is large enough so that the time required by the acid pickling treatment is largely reduced. When Rem is added in the ladle as in the paragraph (2) of the present invention, good uniformity of the Rem addition in the steel is assured, and in case of the paragraph (3) of the present invention, a remarkably good yield of Rem is assured.

Examples of the present invention will be described hereinunder.

Molten steel prepared in a converter, having a ladle analysis showing 0.018% of carbon and 0.17% of manganese was subjected to vacuum degassing and predeoxidized by addition of aluminum. In case of the steel A, the molten steel was left one minute after casting into an ingot mold to form an initial solidified layer in the ingot, and titanium and Rem were added to the non-solidified portion. In case of the steel B, Rem was added simultaneously when the ingot-casting of the molten steel was started, and the molten steel was left 1 minute after the casting to form an initial solidified layer, and titanium was added to the non-solidified portion. In case of the steel C, titanium and Rem were added simultaneously when the ingot-casting was started.

In case. of the comparative steels D and E, the molten steel was left one minute after the ingot-casting to form an initial solidified layer in the steel ingot, and titanium was added to the non-solidified layer (steel D) and Rem was added to the non-solidified layer (steel E). All of the inventive steels A, B, and C and the comparative steels D and E were hot rolled to 6.5mm thickness with a hot coiling temperature of 590C, cold rolled to 2.0mm thickness, and subjected to a box annealing at 700C for 12 hours and 1% temper rolling.

The chemical analyses of the steel sheets thus produced are shown in Table 1 and the mechanical properties are shown in Table 2. The thickness of the initial solidified layer of the inventive steel A is shown in FIG. 1. As understood from Table 2, the inventive steels A,

B, and C show excellent deep-drawability but does not show aging at all. Also these steel sheets were subjected to enamel firing with single application of glaze using glaze which easily causes fish-scale, and fish-scale occurrence was investigated.

It has been confirmed from the results shown in Table 4 that the inventive steels A, B, and C show far better resistance against the occurrence of fish-scale as compared with the comparative steel D in which only titanium is added and the steel E in which only Rem is added. This remarkable results are obtained by the combined addition of titanium and Rem, and it has Table 1.

Chemical Analysis of Steel Sheets by weight) Total Total C Mn Si P S 0 Ti Rem Ce N Remarks Inventive A 0.004 0.17 0.02 0.011 0.005 0.0030 0.049 0.105 0.030 0.0069 with initial solidified layer Inventive B 0.004 0.17 0.02 0.012 0.006 0.0020 0.043 0.095 0.035 0.0070

(Rem is contained in initial solidified layer) Inventive 0.004 0.17 0.02 0.011 0.007 0.0040 0.048 0.115 0.052 0.0066 with no initial solidified layer Comparative 0.005 0.18 0.02 0.011 0.009 0.0030 0.046 0.0060 with initial solidified layer Comparative 0.003 0.17 0.02 0.011 0.007 0.0050 0.093 0.038 0.0086

Table 2:

Mechanical Properties of" Steel Sheet (2.0mm thickness) Yield point elongation YP TS El T 100C X 60 minutes aging Inventive A 16.0 28.8 55 1.95 0.26 0 Inventive B 16.2 28.1 56 1.90 0.26 0 Inventive C 15.3 28.2 54 1.80 0.26 0 Comparative D 13.4 29.8 54 1.92 0.26 0 Comparative E 23.4 30.8 54 1.36 0.21 3.0

been discovered through various studies that same or similar results can not be obtained if titanium and other Table inclusions are present simultaneously. As another Weight method for determining the fish-scale resistance, meafig fi z surements were made on hydrogen permeation times. According to this method, electrolysis was effected on Inventive Steel one side of the steel sheet, and the time required by A 680 hydrogen for diffusing the steel sheet and reaching the B 675 other side of the steel sheet (this time called the hydro- C 342 gen permeation time) was measured. As longer hydrogen permeation time represents larger ability of absorbing hydrogen within the steel sheet, and therefore bet- Table 4. ter fish-scale resistance. The results of measurements at Fismcale 25C of the hydrogen permeation times of the steels A, occurrence B, C, D and E according to the method are shown in Table 5, from which it is clear that the inventive steels Inventive Steel A, B and C show a remarkably long hydrogen perme- Q 8 ation time. It has been also confirmed that the inventive c 0 steels A, B, and C were completely free from defects COmPaWSiVe Steel 32 such as foams and blisters after the enamel firing with E 40 single application of glaze, and showed very good adhesion.

Next, the inventive steels A, B, and C were immersed T bl 5 in 10% H 80 solution at 70C for 10 minutes and a 6 measured for weight decrease by acid pickling. The Inventive S e Hyd g Permeation Time results are shown in Table 3. The inventive steels A and A 168 B showed a remarkably large weight decrease and were g 13 7 found to be very advantageous for efficiency of enamel Comparative Stee] firing. Also the surface of the mventive steels A and B D 24,5

What is claimed is:

1. A process for producing an enamelling steel sheet which comprises tapping molten steel with a manganese content of not more than 0.40%, lowering the carbon content in the molten steel to not more than 0.01%, adding titanium to the molten steel in an amount enough to obtain a titanium content between 0.01 and 0.3%, and adding one or more elements selected from the group consisting of the rare earth elements in a total amount between 0.01 and 0.15%, casting the molten steel into ingot or slab, hot rolling and then cold rolling the ingot or slab and then annealing the steel sheet.

2. The process of claim 1 wherein the amount of titanium is between 0.03 and 0.1%.

3. The process of claim 1 wherein the rare earth element is lanthanum, cerium, neodymium, praseodymium or samarium.

4. The process of claim 1 wherein the amount of rare earth element is between 0.02 and 0.12%.

5. The process of claim 1 wherein the cold rolling rate is between 30 and 90%.

6. A process for producing an enamelling steel sheet which comprises tapping molten steel with a manganese content ot not more than 0.40%, lowering the carbon content in the molten steel to not more than 0.01% by vacuum degassing, adding one or more elements selected from the group consisting of the rare earth elements to the steel in a total amount between 0.01 and 0.15%, prior to or at the same time of casting the steel into an ingot mold, leaving the steel after casting in the mold until an initial solidified layer is formed, adding titanium to the non-solidified portion so as to obtain a titanium content between 0.01 and 0.3%, and hot rolling, cold rolling and annealing the steel sheet.

7. The process of claim 6, wherein the amount of rare earth element is between 0.02 and 0.12%.

8. The process of claim 6, wherein the amount of titanium is between 0.03 and 0.1%.

9. A process for producing an enamelling steel sheet which comprises tapping molten steel with a manganese content of not more than 0.40%, lowering the carbon content in the steel to not more than 0.01% by vacuum degassing, leaving the steel after casting in an ingot mold until an initial solidified layer is formed, adding titanium and an element selected from the group consisting of one or more of the rare earth elements to the non-solidified portion in an amount to obtain a titanium content between 0.01 and 0.3%, and a total content of the rare earth elements between 0.01 and 0.15%, casting the steel into an ingot, hot rolling and cold rolling the steel, and then annealing the steel sheet.

10. The process of claim 9, wherein the amount of titanium is between 0.03 and 0.1%.

11. The process of claim 9, wherein the amount of rare earth element is between 0.02 and 0.12%.

12. The process of claim 9, wherein the cold rolling rate is between 30 and 

1. A PROCESS FOR PRODUCING AN ENAMELLING STEEL SHEET WHICH COMPRISING TAPPING MOLTEN STEEL WITH A MANGANESE CONTENT OF NOT MORE THAN 0.40%, LOWERING THE CARBON CONTENT IN THE MOLTEN STEEL TO NOT MORE THAN 0.01%, ADDING TITANIUM TO THE MOLEN STEEL IN AN AMOUNT ENOUGH TO OBTAIN A TITANIUM CONTENT BETWEEN 0.01 AND 0.3%, AND ADDING ONE OR MORE ELEMENTS SELECTED FROM THE GROUP CONSISTING OF THE RARE EARTH ELEMENTS IN A TOTAL AMOUNT BETWEEN 0.01 AND 0.15%, CASTING THE MOLTEN STEEL INTO INGOT OR SLAB, HOT ROLLING AND THEN COLD ROLLING THE INGOT OF SLAB AND THEN ANNEALING THE STEEL SHEET.
 2. The process of claim 1 wherein the amount of titanium is between 0.03 and 0.1%.
 3. The process of claim 1 wherein the rare earth element is lanthanum, cerium, neodymium, praseodymium or samarium.
 4. The process of claim 1 wherein the amount of rare earth element is between 0.02 and 0.12%.
 5. The process of claim 1 wherein the cold rolling rate is between 30 and 90%.
 6. A process for producing an enamelling steel sheet which comprises tapping molten steel with a manganese content ot not more than 0.40%, lowering the carbon content in the molten steel to not more than 0.01% by vacuum degassing, adding one or more elements selected from the group consisting of the rare earth elements to the steel in a total amount between 0.01 and 0.15%, prior to or at the same time of casting the steel into an ingot mold, leaving the steel after casting in the mold until an initial solidified layer is formed, adding titanium to the non-solidified portion so as to obtain a titanium content between 0.01 and 0.3%, and hot rolling, cold rolling and annealing the steel sheet.
 7. The process of claim 6, wherein the amount of rare earth element is between 0.02 and 0.12%.
 8. The process of claim 6, wherein the amount of titanium is between 0.03 and 0.1%.
 9. A process for producing an enamelling steel sheet which comprises tapping molten steel with a manganese content of not more than 0.40%, lowering the carbon content in the steel to not more than 0.01% by vacuum degassing, leaving the steel after casting in an ingot mold until an initial solidified layer is formed, adding titanium and an element selected from the group consisting of one or more of the rare earth elements to the non-solidified portion in an amount to obtain a titanium content between 0.01 and 0.3%, and a total content of the rare earth elements between 0.01 and 0.15%, casting the steel into an ingot, hot rolling and cold rolling the steel, and then annealing the steel sheet.
 10. The process of claim 9, wherein the amount of titanium Is between 0.03 and 0.1%.
 11. The process of claim 9, wherein the amount of rare earth element is between 0.02 and 0.12%.
 12. The process of claim 9, wherein the cold rolling rate is between 30 and 90%. 